Add/edit cubids.py docstrings

[singlesp]

Closes #358 and closes #384.

Changes proposed in this pull request

• add/edit docstrings to cubids.py

[singlesp]

fixed docstrings on entity key but that broke build path doctests and a bunch of other stuff from there. Need to update and revert back to ba31cca and delete doctests in get_entity_value and img_to_new_ext

[singlesp]

orig doc test was failing due to path/to file in the doctest not containing the ses and sub folders. That means this function as written previously will only work for ses and sub.

try using this for get entity value:

from bids.layout import parse_file_entities

[singlesp]

after trying to make get_entity_values more generalizable, I ultimately came to the conclusion that this function was indeed written for this very specific scenario.